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Network Working Group C. Kalt
Request for Comments: 2813 April 2000
Updates: 1459
Category: Informational
Internet Relay Chat: Server Protocol
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
While based on the client-server model, the IRC (Internet Relay Chat)
protocol allows servers to connect to each other effectively forming
a network.
This document defines the protocol used by servers to talk to each
other. It was originally a superset of the client protocol but has
evolved differently.
First formally documented in May 1993 as part of RFC 1459 [IRC], most
of the changes brought since then can be found in this document as
development was focused on making the protocol scale better. Better
scalability has allowed existing world-wide networks to keep growing
and reach sizes which defy the old specification.
Table of Contents
1. Introduction ............................................... 3
2. Global database ............................................ 3
2.1 Servers ................................................ 3
2.2 Clients ................................................ 4
2.2.1 Users ............................................. 4
2.2.2 Services .......................................... 4
2.3 Channels ............................................... 4
3. The IRC Server Specification ............................... 5
3.1 Overview ............................................... 5
3.2 Character codes ........................................ 5
3.3 Messages ............................................... 5
3.3.1 Message format in Augmented BNF ................... 6
3.4 Numeric replies ........................................ 7
4. Message Details ............................................ 7
4.1 Connection Registration ................................ 8
4.1.1 Password message .................................. 8
4.1.2 Server message .................................... 9
4.1.3 Nick .............................................. 10
4.1.4 Service message ................................... 11
4.1.5 Quit .............................................. 12
4.1.6 Server quit message ............................... 13
4.2 Channel operations ..................................... 14
4.2.1 Join message ...................................... 14
4.2.2 Njoin message ..................................... 15
4.2.3 Mode message ...................................... 16
5. Implementation details .................................... 16
5.1 Connection 'Liveness' .................................. 16
5.2 Accepting a client to server connection ................ 16
5.2.1 Users ............................................. 16
5.2.2 Services .......................................... 17
5.3 Establishing a server-server connection. ............... 17
5.3.1 Link options ...................................... 17
5.3.1.1 Compressed server to server links ............ 18
5.3.1.2 Anti abuse protections ....................... 18
5.3.2 State information exchange when connecting ........ 18
5.4 Terminating server-client connections .................. 19
5.5 Terminating server-server connections .................. 19
5.6 Tracking nickname changes .............................. 19
5.7 Tracking recently used nicknames ....................... 20
5.8 Flood control of clients ............................... 20
5.9 Non-blocking lookups ................................... 21
5.9.1 Hostname (DNS) lookups ............................ 21
5.9.2 Username (Ident) lookups .......................... 21
6. Current problems ........................................... 21
6.1 Scalability ............................................ 21
6.2 Labels ................................................. 22
6.2.1 Nicknames ......................................... 22
6.2.2 Channels .......................................... 22
6.2.3 Servers ........................................... 22
6.3 Algorithms ............................................. 22
7. Security Considerations .................................... 23
7.1 Authentication ......................................... 23
7.2 Integrity .............................................. 23
8. Current support and availability ........................... 24
9. Acknowledgements ........................................... 24
10. References ................................................ 24
11. Author's Address .......................................... 25
12. Full Copyright Statement ................................... 26
1. Introduction
This document is intended for people working on implementing an IRC
server but will also be useful to anyone implementing an IRC service.
Servers provide the three basic services required for realtime
conferencing defined by the "Internet Relay Chat: Architecture"
[IRC-ARCH]: client locator (via the client protocol [IRC-CLIENT]),
message relaying (via the server protocol defined in this document)
and channel hosting and management (following specific rules [IRC-
CHAN]).
2. Global database
Although the IRC Protocol defines a fairly distributed model, each
server maintains a "global state database" about the whole IRC
network. This database is, in theory, identical on all servers.
2.1 Servers
Servers are uniquely identified by their name which has a maximum
length of sixty three (63) characters. See the protocol grammar
rules (section 3.3.1) for what may and may not be used in a server
name.
Each server is typically known by all other servers, however it is
possible to define a "hostmask" to group servers together according
to their name. Inside the hostmasked area, all the servers have a
name which matches the hostmask, and any other server with a name
matching the hostmask SHALL NOT be connected to the IRC network
outside the hostmasked area. Servers which are outside the area have
no knowledge of the individual servers present inside the area,
instead they are presented with a virtual server which has the
hostmask for name.
2.2 Clients
For each client, all servers MUST have the following information: a
netwide unique identifier (whose format depends on the type of
client) and the server to which the client is connected.
2.2.1 Users
Each user is distinguished from other users by a unique nickname
having a maximum length of nine (9) characters. See the protocol
grammar rules (section 3.3.1) for what may and may not be used in a
nickname. In addition to the nickname, all servers MUST have the
following information about all users: the name of the host that the
user is running on, the username of the user on that host, and the
server to which the client is connected.
2.2.2 Services
Each service is distinguished from other services by a service name
composed of a nickname and a server name. The nickname has a maximum
length of nine (9) characters. See the protocol grammar rules
(section 3.3.1) for what may and may not be used in a nickname. The
server name used to compose the service name is the name of the
server to which the service is connected. In addition to this
service name all servers MUST know the service type.
Services differ from users by the format of their identifier, but
more importantly services and users don't have the same type of
access to the server: services can request part or all of the global
state information that a server maintains, but have a more restricted
set of commands available to them (See "IRC Client Protocol" [IRC-
CLIENT] for details on which) and are not allowed to join channels.
Finally services are not usually subject to the "Flood control"
mechanism described in section 5.8.
2.3 Channels
Alike services, channels have a scope [IRC-CHAN] and are not
necessarily known to all servers. When a channel existence is known
to a server, the server MUST keep track of the channel members, as
well as the channel modes.
3. The IRC Server Specification
3.1 Overview
The protocol as described herein is for use with server to server
connections. For client to server connections, see the IRC Client
Protocol specification.
There are, however, more restrictions on client connections (which
are considered to be untrustworthy) than on server connections.
3.2 Character codes
No specific character set is specified. The protocol is based on a a
set of codes which are composed of eight (8) bits, making up an
octet. Each message may be composed of any number of these octets;
however, some octet values are used for control codes which act as
message delimiters.
Regardless of being an 8-bit protocol, the delimiters and keywords
are such that protocol is mostly usable from US-ASCII terminal and a
telnet connection.
Because of IRC's Scandinavian origin, the characters {}|^ are
considered to be the lower case equivalents of the characters []\~,
respectively. This is a critical issue when determining the
equivalence of two nicknames, or channel names.
3.3 Messages
Servers and clients send each other messages which may or may not
generate a reply. Most communication between servers do not generate
any reply, as servers mostly perform routing tasks for the clients.
Each IRC message may consist of up to three main parts: the prefix
(OPTIONAL), the command, and the command parameters (maximum of
fifteen (15)). The prefix, command, and all parameters are separated
by one ASCII space character (0x20) each.
The presence of a prefix is indicated with a single leading ASCII
colon character (':', 0x3b), which MUST be the first character of the
message itself. There MUST be NO gap (whitespace) between the colon
and the prefix. The prefix is used by servers to indicate the true
origin of the message. If the prefix is missing from the message, it
is assumed to have originated from the connection from which it was
received. Clients SHOULD not use a prefix when sending a message
from themselves; if they use one, the only valid prefix is the
registered nickname associated with the client.
When a server receives a message, it MUST identify its source using
the (eventually assumed) prefix. If the prefix cannot be found in
the server's internal database, it MUST be discarded, and if the
prefix indicates the message comes from an (unknown) server, the link
from which the message was received MUST be dropped. Dropping a link
in such circumstances is a little excessive but necessary to maintain
the integrity of the network and to prevent future problems. Another
common error condition is that the prefix found in the server's
internal database identifies a different source (typically a source
registered from a different link than from which the message
arrived). If the message was received from a server link and the
prefix identifies a client, a KILL message MUST be issued for the
client and sent to all servers. In other cases, the link from which
the message arrived SHOULD be dropped for clients, and MUST be
dropped for servers. In all cases, the message MUST be discarded.
The command MUST either be a valid IRC command or a three (3) digit
number represented in ASCII text.
IRC messages are always lines of characters terminated with a CR-LF
(Carriage Return - Line Feed) pair, and these messages SHALL NOT
exceed 512 characters in length, counting all characters including
the trailing CR-LF. Thus, there are 510 characters maximum allowed
for the command and its parameters. There is no provision for
continuation message lines. See section 5 for more details about
current implementations.
3.3.1 Message format in Augmented BNF
The protocol messages must be extracted from the contiguous stream of
octets. The current solution is to designate two characters, CR and
LF, as message separators. Empty messages are silently ignored,
which permits use of the sequence CR-LF between messages without
extra problems.
The extracted message is parsed into the components <prefix>,
<command> and list of parameters (<params>).
The Augmented BNF representation for this is found in "IRC Client
Protocol" [IRC-CLIENT].
The extended prefix (["!" user "@" host ]) MUST NOT be used in server
to server communications and is only intended for server to client
messages in order to provide clients with more useful information
about who a message is from without the need for additional queries.
3.4 Numeric replies
Most of the messages sent to the server generate a reply of some
sort. The most common reply is the numeric reply, used for both
errors and normal replies. The numeric reply MUST be sent as one
message consisting of the sender prefix, the three digit numeric, and
the target of the reply. A numeric reply is not allowed to originate
from a client; any such messages received by a server are silently
dropped. In all other respects, a numeric reply is just like a normal
message, except that the keyword is made up of 3 numeric digits
rather than a string of letters. A list of different replies is
supplied in "IRC Client Protocol" [IRC-CLIENT].
4. Message Details
All the messages recognized by the IRC server and client are
described in the IRC Client Protocol specification.
Where the reply ERR_NOSUCHSERVER is returned, it means that the
target of the message could not be found. The server MUST NOT send
any other replies after this error for that command.
The server to which a client is connected is required to parse the
complete message, returning any appropriate errors. If the server
encounters a fatal error while parsing a message, an error MUST be
sent back to the client and the parsing terminated. A fatal error
may follow from incorrect command, a destination which is otherwise
unknown to the server (server, client or channel names fit this
category), not enough parameters or incorrect privileges.
If a full set of parameters is presented, then each MUST be checked
for validity and appropriate responses sent back to the client. In
the case of messages which use parameter lists using the comma as an
item separator, a reply MUST be sent for each item.
In the examples below, some messages appear using the full format:
:Name COMMAND parameter list
Such examples represent a message from "Name" in transit between
servers, where it is essential to include the name of the original
sender of the message so remote servers may send back a reply along
the correct path.
The message details for client to server communication are described
in the "IRC Client Protocol" [IRC-CLIENT]. Some sections in the
following pages apply to some of these messages, they are additions
to the message specifications which are only relevant to server to
server communication, or to the server implementation. The messages
which are introduced here are only used for server to server
communication.
4.1 Connection Registration
The commands described here are used to register a connection with
another IRC server.
4.1.1 Password message
Command: PASS
Parameters: <password> <version> <flags> [<options>]
The PASS command is used to set a 'connection password'. The
password MUST be set before any attempt to register the connection is
made. Currently this means that servers MUST send a PASS command
before any SERVER command. Only one (1) PASS command SHALL be
accepted from a connection.
The last three (3) parameters MUST be ignored if received from a
client (e.g. a user or a service). They are only relevant when
received from a server.
The <version> parameter is a string of at least four (4) characters,
and up to fourteen (14) characters. The first four (4) characters
MUST be digits and indicate the protocol version known by the server
issuing the message. The protocol described by this document is
version 2.10 which is encoded as "0210". The remaining OPTIONAL
characters are implementation dependent and should describe the
software version number.
The <flags> parameter is a string of up to one hundred (100)
characters. It is composed of two substrings separated by the
character "|" (%x7C). If present, the first substring MUST be the
name of the implementation. The reference implementation (See
Section 8, "Current support and availability") uses the string "IRC".
If a different implementation is written, which needs an identifier,
then that identifier should be registered through publication of an
RFC. The second substring is implementation dependent. Both
substrings are OPTIONAL, but the character "|" is REQUIRED. The
character "|" MUST NOT appear in either substring.
Finally, the last parameter, <options>, is used for link options.
The only options defined by the protocol are link compression (using
the character "Z"), and an abuse protection flag (using the character
"P"). See sections 5.3.1.1 (Compressed server to server links) and
5.3.1.2 (Anti abuse protections) respectively for more information on
these options.
Numeric Replies:
ERR_NEEDMOREPARAMS ERR_ALREADYREGISTRED
Example:
PASS moresecretpassword 0210010000 IRC|aBgH$ Z
4.1.2 Server message
Command: SERVER
Parameters: <servername> <hopcount> <token> <info>
The SERVER command is used to register a new server. A new connection
introduces itself as a server to its peer. This message is also used
to pass server data over whole net. When a new server is connected
to net, information about it MUST be broadcasted to the whole
network.
The <info> parameter may contain space characters.
<hopcount> is used to give all servers some internal information on
how far away each server is. Local peers have a value of 0, and each
passed server increments the value. With a full server list, it
would be possible to construct a map of the entire server tree, but
hostmasks prevent this from being done.
The <token> parameter is an unsigned number used by servers as an
identifier. This identifier is subsequently used to reference a
server in the NICK and SERVICE messages sent between servers. Server
tokens only have a meaning for the point-to-point peering they are
used and MUST be unique for that connection. They are not global.
The SERVER message MUST only be accepted from either (a) a connection
which is yet to be registered and is attempting to register as a
server, or (b) an existing connection to another server, in which
case the SERVER message is introducing a new server behind that
server.
Most errors that occur with the receipt of a SERVER command result in
the connection being terminated by the destination host (target
SERVER). Because of the severity of such event, error replies are
usually sent using the "ERROR" command rather than a numeric.
If a SERVER message is parsed and it attempts to introduce a server
which is already known to the receiving server, the connection, from
which that message arrived, MUST be closed (following the correct
procedures), since a duplicate route to a server has been formed and
the acyclic nature of the IRC tree breaks. In some conditions, the
connection from which the already known server has registered MAY be
closed instead. It should be noted that this kind of error can also
be the result of a second running server, problem which cannot be
fixed within the protocol and typically requires human intervention.
This type of problem is particularly insidious, as it can quite
easily result in part of the IRC network to be isolated, with one of
the two servers connected to each partition therefore making it
impossible for the two parts to unite.
Numeric Replies:
ERR_ALREADYREGISTRED
Example:
SERVER test.oulu.fi 1 1 :Experimental server ; New server
test.oulu.fi introducing itself and
attempting to register.
:tolsun.oulu.fi SERVER csd.bu.edu 5 34 :BU Central Server ; Server
tolsun.oulu.fi is our uplink for
csd.bu.edu which is 5 hops away. The
token "34" will be used by
tolsun.oulu.fi when introducing new
users or services connected to
csd.bu.edu.
4.1.3 Nick
Command: NICK
Parameters: <nickname> <hopcount> <username> <host> <servertoken>
<umode> <realname>
This form of the NICK message MUST NOT be allowed from user
connections. However, it MUST be used instead of the NICK/USER pair
to notify other servers of new users joining the IRC network.
This message is really the combination of three distinct messages:
NICK, USER and MODE [IRC-CLIENT].
The <hopcount> parameter is used by servers to indicate how far away
a user is from its home server. A local connection has a hopcount of
0. The hopcount value is incremented by each passed server.
The <servertoken> parameter replaces the <servername> parameter of
the USER (See section 4.1.2 for more information on server tokens).
Examples:
NICK syrk 5 kalt millennium.stealth.net 34 +i :Christophe Kalt ; New
user with nickname "syrk", username
"kalt", connected from host
"millennium.stealth.net" to server
"34" ("csd.bu.edu" according to the
previous example).
:krys NICK syrk ; The other form of the NICK message,
as defined in "IRC Client Protocol"
[IRC-CLIENT] and used between
servers: krys changed his nickname to
syrk
4.1.4 Service message
Command: SERVICE
Parameters: <servicename> <servertoken> <distribution> <type>
<hopcount> <info>
The SERVICE command is used to introduce a new service. This form of
the SERVICE message SHOULD NOT be allowed from client (unregistered,
or registered) connections. However, it MUST be used between servers
to notify other servers of new services joining the IRC network.
The <servertoken> is used to identify the server to which the service
is connected. (See section 4.1.2 for more information on server
tokens).
The <hopcount> parameter is used by servers to indicate how far away
a service is from its home server. A local connection has a hopcount
of 0. The hopcount value is incremented by each passed server.
The <distribution> parameter is used to specify the visibility of a
service. The service may only be known to servers which have a name
matching the distribution. For a matching server to have knowledge
of the service, the network path between that server and the server
to which the service is connected MUST be composed of servers whose
names all match the mask. Plain "*" is used when no restriction is
wished.
The <type> parameter is currently reserved for future usage.
Numeric Replies:
ERR_ALREADYREGISTRED ERR_NEEDMOREPARAMS
ERR_ERRONEUSNICKNAME
RPL_YOURESERVICE RPL_YOURHOST
RPL_MYINFO
Example:
SERVICE dict@irc.fr 9 *.fr 0 1 :French Dictionary r" registered on
server "9" is being announced to
another server. This service will
only be available on servers whose
name matches "*.fr".
4.1.5 Quit
Command: QUIT
Parameters: [<Quit Message>]
A client session ends with a quit message. The server MUST close the
connection to a client which sends a QUIT message. If a "Quit
Message" is given, this will be sent instead of the default message,
the nickname or service name.
When "netsplit" (See Section 4.1.6) occur, the "Quit Message" is
composed of the names of two servers involved, separated by a space.
The first name is that of the server which is still connected and the
second name is either that of the server which has become
disconnected or that of the server to which the leaving client was
connected:
<Quit Message> = ":" servername SPACE servername
Because the "Quit Message" has a special meaning for "netsplits",
servers SHOULD NOT allow a client to use a <Quit Message> in the
format described above.
If, for some other reason, a client connection is closed without the
client issuing a QUIT command (e.g. client dies and EOF occurs on
socket), the server is REQUIRED to fill in the quit message with some
sort of message reflecting the nature of the event which caused it to
happen. Typically, this is done by reporting a system specific
error.
Numeric Replies:
None.
Examples:
:WiZ QUIT :Gone to have lunch ; Preferred message format.
4.1.6 Server quit message
Command: SQUIT
Parameters: <server> <comment>
The SQUIT message has two distinct uses.
The first one (described in "Internet Relay Chat: Client Protocol"
[IRC-CLIENT]) allows operators to break a local or remote server
link. This form of the message is also eventually used by servers to
break a remote server link.
The second use of this message is needed to inform other servers when
a "network split" (also known as "netsplit") occurs, in other words
to inform other servers about quitting or dead servers. If a server
wishes to break the connection to another server it MUST send a SQUIT
message to the other server, using the name of the other server as
the server parameter, which then closes its connection to the
quitting server.
The <comment> is filled in by servers which SHOULD place an error or
similar message here.
Both of the servers which are on either side of the connection being
closed are REQUIRED to send out a SQUIT message (to all its other
server connections) for all other servers which are considered to be
behind that link.
Similarly, a QUIT message MAY be sent to the other still connected
servers on behalf of all clients behind that quitting link. In
addition to this, all channel members of a channel which lost a
member due to the "split" MUST be sent a QUIT message. Messages to
channel members are generated by each client's local server.
If a server connection is terminated prematurely (e.g., the server on
the other end of the link died), the server which detects this
disconnection is REQUIRED to inform the rest of the network that the
connection has closed and fill in the comment field with something
appropriate.
When a client is removed as the result of a SQUIT message, the server
SHOULD add the nickname to the list of temporarily unavailable
nicknames in an attempt to prevent future nickname collisions. See
section 5.7 (Tracking recently used nicknames) for more information
on this procedure.
Numeric replies:
ERR_NOPRIVILEGES ERR_NOSUCHSERVER
ERR_NEEDMOREPARAMS
Example:
SQUIT tolsun.oulu.fi :Bad Link ? ; the server link tolson.oulu.fi
has been terminated because of "Bad
Link".
:Trillian SQUIT cm22.eng.umd.edu :Server out of control ; message
from Trillian to disconnect
"cm22.eng.umd.edu" from the net
because "Server out of control".
4.2 Channel operations
This group of messages is concerned with manipulating channels, their
properties (channel modes), and their contents (typically users). In
implementing these, a number of race conditions are inevitable when
users at opposing ends of a network send commands which will
ultimately clash. It is also REQUIRED that servers keep a nickname
history to ensure that wherever a <nick> parameter is given, the
server check its history in case it has recently been changed.
4.2.1 Join message
Command: JOIN
Parameters: <channel>[ %x7 <modes> ]
*( "," <channel>[ %x7 <modes> ] )
The JOIN command is used by client to start listening a specific
channel. Whether or not a client is allowed to join a channel is
checked only by the local server the client is connected to; all
other servers automatically add the user to the channel when the
command is received from other servers.
Optionally, the user status (channel modes 'O', 'o', and 'v') on the
channel may be appended to the channel name using a control G (^G or
ASCII 7) as separator. Such data MUST be ignored if the message
wasn't received from a server. This format MUST NOT be sent to
clients, it can only be used between servers and SHOULD be avoided.
The JOIN command MUST be broadcast to all servers so that each server
knows where to find the users who are on the channel. This allows
optimal delivery of PRIVMSG and NOTICE messages to the channel.
Numeric Replies:
ERR_NEEDMOREPARAMS ERR_BANNEDFROMCHAN
ERR_INVITEONLYCHAN ERR_BADCHANNELKEY
ERR_CHANNELISFULL ERR_BADCHANMASK
ERR_NOSUCHCHANNEL ERR_TOOMANYCHANNELS
ERR_TOOMANYTARGETS ERR_UNAVAILRESOURCE
RPL_TOPIC
Examples:
:WiZ JOIN #Twilight_zone ; JOIN message from WiZ
4.2.2 Njoin message
Command: NJOIN
Parameters: <channel> [ "@@" / "@" ] [ "+" ] <nickname>
*( "," [ "@@" / "@" ] [ "+" ] <nickname> )
The NJOIN message is used between servers only. If such a message is
received from a client, it MUST be ignored. It is used when two
servers connect to each other to exchange the list of channel members
for each channel.
Even though the same function can be performed by using a succession
of JOIN, this message SHOULD be used instead as it is more efficient.
The prefix "@@" indicates that the user is the "channel creator", the
character "@" alone indicates a "channel operator", and the character
'+' indicates that the user has the voice privilege.
Numeric Replies:
ERR_NEEDMOREPARAMS ERR_NOSUCHCHANNEL
ERR_ALREADYREGISTRED
Examples:
:ircd.stealth.net NJOIN #Twilight_zone :@WiZ,+syrk,avalon ; NJOIN
message from ircd.stealth.net
announcing users joining the
#Twilight_zone channel: WiZ with
channel operator status, syrk with
voice privilege and avalon with no
privilege.
4.2.3 Mode message
The MODE message is a dual-purpose command in IRC. It allows both
usernames and channels to have their mode changed.
When parsing MODE messages, it is RECOMMENDED that the entire message
be parsed first, and then the changes which resulted passed on.
It is REQUIRED that servers are able to change channel modes so that
"channel creator" and "channel operators" may be created.
5. Implementation details
A the time of writing, the only current implementation of this
protocol is the IRC server, version 2.10. Earlier versions may
implement some or all of the commands described by this document with
NOTICE messages replacing many of the numeric replies. Unfortunately,
due to backward compatibility requirements, the implementation of
some parts of this document varies with what is laid out. One
notable difference is:
* recognition that any LF or CR anywhere in a message marks
the end of that message (instead of requiring CR-LF);
The rest of this section deals with issues that are mostly of
importance to those who wish to implement a server but some parts
also apply directly to clients as well.
5.1 Connection 'Liveness'
To detect when a connection has died or become unresponsive, the
server MUST poll each of its connections. The PING command (See "IRC
Client Protocol" [IRC-CLIENT]) is used if the server doesn't get a
response from its peer in a given amount of time.
If a connection doesn't respond in time, its connection is closed
using the appropriate procedures.
5.2 Accepting a client to server connection
5.2.1 Users
When a server successfully registers a new user connection, it is
REQUIRED to send to the user unambiguous messages stating: the user
identifiers upon which it was registered (RPL_WELCOME), the server
name and version (RPL_YOURHOST), the server birth information
(RPL_CREATED), available user and channel modes (RPL_MYINFO), and it
MAY send any introductory messages which may be deemed appropriate.
In particular the server SHALL send the current user/service/server
count (as per the LUSER reply) and finally the MOTD (if any, as per
the MOTD reply).
After dealing with registration, the server MUST then send out to
other servers the new user's nickname (NICK message), other
information as supplied by itself (USER message) and as the server
could discover (from DNS servers). The server MUST NOT send this
information out with a pair of NICK and USER messages as defined in
"IRC Client Protocol" [IRC-CLIENT], but MUST instead take advantage
of the extended NICK message defined in section 4.1.3.
5.2.2 Services
Upon successfully registering a new service connection, the server is
subject to the same kind of REQUIREMENTS as for a user. Services
being somewhat different, only the following replies are sent:
RPL_YOURESERVICE, RPL_YOURHOST, RPL_MYINFO.
After dealing with this, the server MUST then send out to other
servers (SERVICE message) the new service's nickname and other
information as supplied by the service (SERVICE message) and as the
server could discover (from DNS servers).
5.3 Establishing a server-server connection.
The process of establishing a server-to-server connection is fraught
with danger since there are many possible areas where problems can
occur - the least of which are race conditions.
After a server has received a connection following by a PASS/SERVER
pair which were recognized as being valid, the server SHOULD then
reply with its own PASS/SERVER information for that connection as
well as all of the other state information it knows about as
described below.
When the initiating server receives a PASS/SERVER pair, it too then
checks that the server responding is authenticated properly before
accepting the connection to be that server.
5.3.1 Link options
Server links are based on a common protocol (defined by this
document) but a particular link MAY set specific options using the
PASS message (See Section 4.1.1).
5.3.1.1 Compressed server to server links
If a server wishes to establish a compressed link with its peer, it
MUST set the 'Z' flag in the options parameter to the PASS message.
If both servers request compression and both servers are able to
initialize the two compressed streams, then the remainder of the
communication is to be compressed. If any server fails to initialize
the stream, it will send an uncompressed ERROR message to its peer
and close the connection.
The data format used for the compression is described by RFC 1950
[ZLIB], RFC 1951 [DEFLATE] and RFC 1952 [GZIP].
5.3.1.2 Anti abuse protections
Most servers implement various kinds of protections against possible
abusive behaviours from non trusted parties (typically users). On
some networks, such protections are indispensable, on others they are
superfluous. To require that all servers implement and enable such
features on a particular network, the 'P' flag is used when two
servers connect. If this flag is present, it means that the server
protections are enabled, and that the server REQUIRES all its server
links to enable them as well.
Commonly found protections are described in sections 5.7 (Tracking
recently used nicknames) and 5.8 (Flood control of clients).
5.3.2 State information exchange when connecting
The order of state information being exchanged between servers is
essential. The REQUIRED order is as follows:
* all known servers;
* all known client information;
* all known channel information.
Information regarding servers is sent via extra SERVER messages,
client information with NICK and SERVICE messages and channels with
NJOIN/MODE messages.
NOTE: channel topics SHOULD NOT be exchanged here because the TOPIC
command overwrites any old topic information, so at best, the two
sides of the connection would exchange topics.
By passing the state information about servers first, any collisions
with servers that already exist occur before nickname collisions
caused by a second server introducing a particular nickname. Due to
the IRC network only being able to exist as an acyclic graph, it may
be possible that the network has already reconnected in another
location. In this event, the place where the server collision occurs
indicates where the net needs to split.
5.4 Terminating server-client connections
When a client connection unexpectedly closes, a QUIT message is
generated on behalf of the client by the server to which the client
was connected. No other message is to be generated or used.
5.5 Terminating server-server connections
If a server-server connection is closed, either via a SQUIT command
or "natural" causes, the rest of the connected IRC network MUST have
its information updated by the server which detected the closure.
The terminating server then sends a list of SQUITs (one for each
server behind that connection). (See Section 4.1.6 (SQUIT)).
5.6 Tracking nickname changes
All IRC servers are REQUIRED to keep a history of recent nickname
changes. This is important to allow the server to have a chance of
keeping in touch of things when nick-change race conditions occur
with commands manipulating them. Messages which MUST trace nick
changes are:
* KILL (the nick being disconnected)
* MODE (+/- o,v on channels)
* KICK (the nick being removed from channel)
No other commands need to check nick changes.
In the above cases, the server is required to first check for the
existence of the nickname, then check its history to see who that
nick now belongs to (if anyone!). This reduces the chances of race
conditions but they can still occur with the server ending up
affecting the wrong client. When performing a change trace for an
above command it is RECOMMENDED that a time range be given and
entries which are too old ignored.
For a reasonable history, a server SHOULD be able to keep previous
nickname for every client it knows about if they all decided to
change. This size is limited by other factors (such as memory, etc).
5.7 Tracking recently used nicknames
This mechanism is commonly known as "Nickname Delay", it has been
proven to significantly reduce the number of nickname collisions
resulting from "network splits"/reconnections as well as abuse.
In addition of keeping track of nickname changes, servers SHOULD keep
track of nicknames which were recently used and were released as the
result of a "network split" or a KILL message. These nicknames are
then unavailable to the server local clients and cannot be re-used
(even though they are not currently in use) for a certain period of
time.
The duration for which a nickname remains unavailable SHOULD be set
considering many factors among which are the size (user wise) of the
IRC network, and the usual duration of "network splits". It SHOULD
be uniform on all servers for a given IRC network.
5.8 Flood control of clients
With a large network of interconnected IRC servers, it is quite easy
for any single client attached to the network to supply a continuous
stream of messages that result in not only flooding the network, but
also degrading the level of service provided to others. Rather than
require every 'victim' to provide their own protection, flood
protection was written into the server and is applied to all clients
except services. The current algorithm is as follows:
* check to see if client's `message timer' is less than current time
(set to be equal if it is);
* read any data present from the client;
* while the timer is less than ten (10) seconds ahead of the current
time, parse any present messages and penalize the client by two (2)
seconds for each message;
* additional penalties MAY be used for specific commands which
generate a lot of traffic across the network.
This in essence means that the client may send one (1) message every
two (2) seconds without being adversely affected. Services MAY also
be subject to this mechanism.
5.9 Non-blocking lookups
In a real-time environment, it is essential that a server process
does as little waiting as possible so that all the clients are
serviced fairly. Obviously this requires non-blocking IO on all
network read/write operations. For normal server connections, this
was not difficult, but there are other support operations that may
cause the server to block (such as disk reads). Where possible, such
activity SHOULD be performed with a short timeout.
5.9.1 Hostname (DNS) lookups
Using the standard resolver libraries from Berkeley and others has
meant large delays in some cases where replies have timed out. To
avoid this, a separate set of DNS routines were written for the
current implementation. Routines were setup for non-blocking IO
operations with local cache, and then polled from within the main
server IO loop.
5.9.2 Username (Ident) lookups
Although there are numerous ident libraries (implementing the
"Identification Protocol" [IDENT]) for use and inclusion into other
programs, these caused problems since they operated in a synchronous
manner and resulted in frequent delays. Again the solution was to
write a set of routines which would cooperate with the rest of the
server and work using non-blocking IO.
6. Current problems
There are a number of recognized problems with this protocol, all of
which are hoped to be solved sometime in the near future during its
rewrite. Currently, work is underway to find working solutions to
these problems.
6.1 Scalability
It is widely recognized that this protocol does not scale
sufficiently well when used in a large arena. The main problem comes
from the requirement that all servers know about all other servers
and clients and that information regarding them be updated as soon as
it changes. It is also desirable to keep the number of servers low
so that the path length between any two points is kept minimal and
the spanning tree as strongly branched as possible.
6.2 Labels
The current IRC protocol has 4 types of labels: the nickname, the
channel name, the server name and the service name. Each of the four
types has its own domain and no duplicates are allowed inside that
domain. Currently, it is possible for users to pick the label for
any of the first three, resulting in collisions. It is widely
recognized that this needs reworking, with a plan for unique names
for nicks that don't collide being desirable as well as a solution
allowing a cyclic tree.
6.2.1 Nicknames
The idea of the nickname on IRC is very convenient for users to use
when talking to each other outside of a channel, but there is only a
finite nickname space and being what they are, it's not uncommon for
several people to want to use the same nick. If a nickname is chosen
by two people using this protocol, either one will not succeed or
both will be removed by use of KILL (See Section 3.7.1 of "IRC Client
Protocol" [IRC-CLIENT]).
6.2.2 Channels
The current channel layout requires that all servers know about all
channels, their inhabitants and properties. Besides not scaling
well, the issue of privacy is also a concern. A collision of
channels is treated as an inclusive event (people from both nets on
channel with common name are considered to be members of it) rather
than an exclusive one such as used to solve nickname collisions.
This protocol defines "Safe Channels" which are very unlikely to be
the subject of a channel collision. Other channel types are kept for
backward compatibility.
6.2.3 Servers
Although the number of servers is usually small relative to the
number of users and channels, they too are currently REQUIRED to be
known globally, either each one separately or hidden behind a mask.
6.3 Algorithms
In some places within the server code, it has not been possible to
avoid N^2 algorithms such as checking the channel list of a set of
clients.
In current server versions, there are only few database consistency
checks, most of the time each server assumes that a neighbouring
server is correct. This opens the door to large problems if a
connecting server is buggy or otherwise tries to introduce
contradictions to the existing net.
Currently, because of the lack of unique internal and global labels,
there are a multitude of race conditions that exist. These race
conditions generally arise from the problem of it taking time for
messages to traverse and effect the IRC network. Even by changing to
unique labels, there are problems with channel-related commands being
disrupted.
7. Security Considerations
7.1 Authentication
Servers only have two means of authenticating incoming connections:
plain text password, and DNS lookups. While these methods are weak
and widely recognized as unsafe, their combination has proven to be
sufficient in the past:
* public networks typically allow user connections with only few
restrictions, without requiring accurate authentication.
* private networks which operate in a controlled environment often
use home-grown authentication mechanisms not available on the
internet: reliable ident servers [IDENT], or other proprietary
mechanisms.
The same comments apply to the authentication of IRC Operators.
It should also be noted that while there has been no real demand over
the years for stronger authentication, and no real effort to provide
better means to safely authenticate users, the current protocol
offers enough to be able to easily plug-in external authentication
methods based on the information that a client can submit to the
server upon connection: nickname, username, password.
7.2 Integrity
Since the PASS and OPER messages of the IRC protocol are sent in
clear text, a stream layer encryption mechanism (like "The TLS
Protocol" [TLS]) could be used to protect these transactions.
8. Current support and availability
Mailing lists for IRC related discussion:
General discussion: ircd-users@irc.org
Protocol development: ircd-dev@irc.org
Software implementations:
ftp://ftp.irc.org/irc/serverftp://ftp.funet.fi/pub/unix/ircftp://coombs.anu.edu.au/pub/irc
Newsgroup: alt.irc
9. Acknowledgements
Parts of this document were copied from the RFC 1459 [IRC] which
first formally documented the IRC Protocol. It has also benefited
from many rounds of review and comments. In particular, the
following people have made significant contributions to this
document:
Matthew Green, Michael Neumayer, Volker Paulsen, Kurt Roeckx, Vesa
Ruokonen, Magnus Tjernstrom, Stefan Zehl.
10. References
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[IRC] Oikarinen, J. and D. Reed, "Internet Relay Chat
Protocol", RFC 1459, May 1993.
[IRC-ARCH] Kalt, C., "Internet Relay Chat: Architecture", RFC 2810,
April 2000.
[IRC-CLIENT] Kalt, C., "Internet Relay Chat: Client Protocol", RFC
2812, April 2000.
[IRC-CHAN] Kalt, C., "Internet Relay Chat: Channel Management", RFC
2811, April 2000.
[ZLIB] Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data
Format Specification version 3.3", RFC 1950, May 1996.
[DEFLATE] Deutsch, P., "DEFLATE Compressed Data Format
Specification version 1.3", RFC 1951, May 1996.
[GZIP] Deutsch, P., "GZIP file format specification version
4.3", RFC 1952, May 1996.
[IDENT] St. Johns, M., "The Identification Protocol", RFC 1413,
February 1993.
[TLS] Dierks, T. and C. Allen, "The TLS Protocol", RFC 2246,
January 1999.
11. Author's Address
Christophe Kalt
99 Teaneck Rd, Apt #117
Ridgefield Park, NJ 07660
USA
EMail: kalt@stealth.net
12. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.

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